Atrial Fibrillation Precursors May Be Novel Stroke Risk Factors. This is a K23 resubmission application for Dr. Hooman Kamel, a neurologist and young investigator pursuing patient-oriented clinical research on ischemic stroke caused by cardiac arrhythmias. A K23 award will provide him with the means to acquire critical skills in three key career development areas: 1) epidemiology and clinical trial design, 2) cardiac diagnostic techniques, and 3) biomarker development and assessment. By acquiring these skills, Dr. Kamel will fulfill his long-term career goal of becoming an independent clinical investigator. To pursue this goal, Dr. Kamel has recruited a primary mentor, Dr. Richard Devereux, a cardiologist with expertise in cardiovascular epidemiology and cardiac diagnostic techniques, and two co- mentors, Dr. Mitchell Elkind, a neurologist with expertise in stroke epidemiology and clinical trial design, and Dr. Costantino Iadecola, a neurologist with expertise in ischemic brain injury and biomarkers. Based on recent evidence and his own preliminary data, Dr. Kamel's central hypothesis is that supraventricular arrhythmias increase stroke risk even before the development of atrial fibrillation/flutter (AF), which is currently the only cardia arrhythmia thought to cause stroke. Testing this hypothesis will address a fundamental gap in knowledge about which supraventricular arrhythmias cause stroke. Until this knowledge gap is filled, optimal strategies for preventing stroke from cardiac disease cannot be fully determined. By pursuing the following specific aims, the applicant will test his hypothesis and gather data for a population- based study of stroke risk from supraventricular arrhythmias (to be proposed in an R01 application during the K23 award period). Specific Aim 1 will test the hypothesis that electrocardiographic P-wave dispersion, an early marker of atrial electrical dysfunction and predisposition to supraventricular arrhythmias, is associated with an increased risk of stroke. This aim will be pursued by analyzing a cohort without AF enrolled in the Strong Heart Study, a population-based epidemiological study with baseline electrocardiographic data and >375 adjudicated and classified cases of stroke. Specific Aim 2 will test the hypothesis that supraventricular ectopy is associated with cryptogenic stroke. In a prospectively enrolled series of patients, rates of supraventricular ectopy during cardiac monitoring will be compared between 75 patients with cryptogenic stroke and 75 patients with stroke from small-vessel occlusion or large-artery atherosclerosis. Secondary analyses will compare biomarkers of cardiac embolism between the two groups, and correlate rates of supraventricular ectopy with these biomarkers. Specific Aim 3 will test the hypothesis that clinical diagnoses of paroxysmal supraventricular tachycardia are associated with the risk of stroke after transient ischemic attack. This aim will b pursued using a validated ICD-9-CM code for paroxysmal supraventricular tachycardia and linked medical records from the California State Inpatient Database and Emergency Department Database. The proposed research is significant because positive results will uncover novel stroke risk factors, while negative results will counter the increasing off-label use of unproven anticoagulant therapy for nonspecific supraventricular arrhythmias after cryptogenic stroke. The proposed research is innovative because it seeks to shift current research and clinical practice paradigms by identifying a large new class of patients who are at high risk for stroke and may benefit from existing anticoagulant drugs, and also seeks to bridge cardiology and neurology via the new application of cardiology tools to stroke research.